It has been known in general that the exhaust gas discharged from the diesel engine contains particulate matter (hereinafter referred to PM) mainly composed of carbon, causing the atmospheric contamination. Accordingly, various devices and methods have been proposed, for trapping the particulate matter and removing it from the exhaust gas.
For example, there have been many proposals wherein the temperature of a diesel particulate filter (DPF) is elevated by forcibly supplying fuel by the injection so that the trapped PM is oxidized and burnt, or NO2 is generated from NO in the exhaust gas and used for oxidizing PM (see Japanese Patent Laid-Open No. 2002-531762, for example), or a catalyzed DPF is used for oxidizing PM (see, for example, Japanese Patent Laid-Open No. H6-272541 and No. H9-125931). There is a problem in the proposals wherein the fuel is forcibly supplied by the injection in that the fuel consumption becomes worse. While, the proposal described in Japanese Patent Laid-Open No. 2002-537162 has a problem in that it is difficult to completely oxidizing PM discharged from the engine and removing the same since the oxidation speed of PM by NO2 is insufficient. In those using the catalyzed DPF described in Japanese Patent Laid-Open No. H6-272541 and No. H9-125931, there is a problem in that, since the catalyst and PM are a solid substance, the both are not sufficiently in contact with each other and result in the incomplete oxidation reaction.
To solve such problems, there has been disclosed (for example, in Japanese Patent Laid-Open No. 2005-502823) a technique for processing PM by the oxidation while using ozone O3 having an oxidization power higher than that of NO2. According to the method and the device for post-processing exhaust gas of the diesel engine disclosed in Japanese Patent Laid-Open No. 2005-502823, a device for generating ozone O3 or nitrogen dioxide NO2 as oxidizing agent from exhaust gas by using plasma is provided upstream from a particulate filter, wherein soot trapped in the particulate filter is oxidized and removed by selectively using ozone and nitrogen dioxide in accordance with the temperature of the exhaust gas, so that when the exhaust gas is at a high temperature, both of ozone and nitrogen dioxide are used, while at a low temperature, nitrogen dioxide is solely used.
In this regard, in the method and the device for post-processing exhaust gas from the diesel engine described in Japanese Patent Laid-Open No. 2005-502823, the improvement in the power for oxidation and removal of PM is recognizable because of the use of ozone O3 higher in oxidation power than NO2. However, there is a risk in that ozone having a high oxidation power may be consumed by the preferential reaction with NOx or HC in the exhaust gas prior to being introduced into the particulate filter, resulting in a problem that an amount of ozone usable for the oxidation and removal of PM becomes less to lower the cleaning efficiency and the oxidation speed of PM.